The invention relates to a method for the identification of gas components in various phases and mixtures of phases of an examination area, in which the gas components can be present dissolved or in gaseous form. The invention also relates to devices for the realization of this method.
For the purpose of a qualitative and quantitative determination of components of a gas in the examination area, that in the following is designated as matrix and can occur as a phase in itself and dissolved in other phases, various methods are known, which make use of the selective permeation of gas components through membranes. Conditions for the quantitative analysis of the concentration of selected known gas components, which are often of interest from a practical point of view, are that the change in the composition of the matrix is caused by changes in concentration of the examined gas components.
Therefore, DE 199 25 842 A1 describes a method, in which the concentration or the partial pressure of gas component contained by a fluid is determined by how quickly the pressure changes.
According to various applications, sensors of different geometries are available, which can be brought into contact with the matrix that is to be examined by minimally interfering with the processes run within the examination area. Thus, a tube-shaped geometry permits the averaging of the concentration along the observed line of laying out in the examination area. According to DE 199 25 842 A1, such a sensor system including a suitable reference sensor system can be installed in a fixed manner within the examination area for the in-situ measurement and does not require any further maintenance.
For membrane-based analysis methods, the used membrane is in contact with the phase containing the matrix on one side, while on the other side a cavity, at least partly enclosed by the membrane, is filled with a gas of known composition. A difference in concentration of gas components on both sides of the membrane causes a diffusive flow of gas molecules through the membrane, and in this way, leads to a change of the partial pressures within the cavity, which can be measured as a change in pressure or volume of the gas phases in the cavity.
The cavity enclosed at least partly by a membrane, which is provided with suitable sensors for measuring pressure or volume or with a measurement value dependent on this measurement, is called a measurement chamber. The measurement chamber in its construction and various components is adapted to the method applied. In this way, the measurement chamber can have if necessary a controllable closing system and a supporting body for the mechanical stabilisation against possible differences in pressure on either side of the membrane. Also, a radial symmetry of the membrane geometry can be used for securing the measurement chamber geometry, whereby the measurement chamber is arranged both on inside and the outside of the radial symmetrical membrane.
Measurement chamber and membrane form a measurement sensor also simply designated as sensor, which if required can be combined with a reference sensor. The latter can have a structure that is analogue to the measurement sensor. A sensor possesses a geometry factor, from which physically relevant geometrical characteristics of the measurement chamber and membrane can be derived as well as a membrane-dependent gas selectivity, which determines how the permeation coefficients of the membrane for the gas components differ from each other. Permeation here is understood as the combination of solution and diffusion of a gas component in the membrane. So far, membranes, which exclusively can be permeated for a gas component, are not known. However, various membranes are distinguished by different sequences of selectivity for individual gas components of the matrix.
The measurement device as described in DE 102 20 944 A1 realizes a spatial segregation between sampling using a sampling element (phase separator) permeable to gases, which is in contact with the materials of the area to be examined on one side, and the sensor system. The latter is based on the sensor system as described in DE 199 25 842 A1. Sensor and sampling element are connected to each other through a circular line permeable to gases. The gas within the circular line can be moved through the circular line, sampling element and sensor permanently or event-initiated using a pump. The change of the pressure within the closed system as a consequence of adjusting the equilibrium of the gas composite within the measurement device through the matrix can serve as an initiating event. Besides the convective conditioning of the measurement chamber via a purge gas, its diffusive conditioning is described via the gas phase circulating through the circular line. For increasing the sensitivity, measurement chambers identical in construction are realized in a differential gas sensor on both sides of the membrane, and the pressure-time behaviour between both measurements chambers are identified using the differential pressure established between both measurement chambers.
The initial state of the measurement, designated by a known gas composite within the measurement chamber and a defined relation between gas and the matrix, can be adjusted to be convective (DE 199 25 842 A1) through the closing system of the measurement chamber, or can be diffusive through the membrane (DE 102 20 944 A1), and is called conditioning.
Under certain conditions, the selectivity of the membrane dependent on a gas component out of equilibrium causes different timelines for re-establishing of the equilibriums on both sides of the membrane. In this way, the measured pressure-time or volume-time curves can be referred back to the original differences in concentrations of one or more gas components. For the given matrix, a component-dependent calibration can be provided for the used sensor in this regard.
If one calculates the different pressure-time curves, which were identified behind membranes of different selectivities, in a system of equations and considers the known initial gas composition in the measurement chambers, it will lead to the theoretically possible and complete analysis of the matrix.
If the matrix composite is of a complex composition, such an analysis requires a high instrumental and technical effort from a practical point of view. Further, there may not be a sufficient amount of membrane materials with sufficient selectivity.
However, the analysis of the matrix itself is often not needed but the analysis of the concentration of a gas component in the matrix is. For instance, this is the concentration or the partial pressure of a qualitatively known gas component, which occurs in the matrix through leakage from a technical device. In an otherwise unamended matrix, this is only possible via a sensor according to DE 199 25 842 A1. Therefore, two different selective sensors are necessary for verification of the concentration for two qualitatively known gas components, which can change independently from each other in the matrix, etc.
Such gas sensors can be used in very different areas, for which the concentrations of the gas components can change. The changes in concentration can have various origins, e.g. transport of material or chemical processes. In this respect, such gas sensors, e.g. used for the monitoring of gas components in waters, soil and rocks, where one must take into consideration biogeochemical processes, or for the monitoring and control of technical devices, disposal sites, reconstruction of contaminated sites, etc.
In particular in regards to gases relevant to the climate, e.g. carbon dioxide (CO2) in relation to the Carbon-Capture-and-Storage-Technology (CCS), monitoring systems, which can be used in a representative and cost efficient manner in-situ are of interest.
In this and similarly in further embodiments, a differentiation of a gas component however can be required in regards to its origin (genesis). Using the example of CO2, it is to be distinguished that CO2 can not only escape from technical or geotechnical devices, e.g. pipelines, grouting drillings or reservoir rock, but also in the relevant monitoring area, e.g. created in soil through metabolic processes. In the first instance, the source is external; in the second instance, it is an internal reaction or soil respiration. A security system should be able to distinguish CCS—CO2 from CO2 as a consequence of soil respiration for such applications. The monitoring of external sources in the monitoring area soil can also be necessary for ascending CO2 as a consequence of smoldering fires or volcanic activity.
For a so called genetic, i.e. origin-based analysis, effortful methods are used to this point, which, for instance, make use of differences in isotope signatures of the various gas sources, which is done with high technical, financial and staff effort. Further, the differences in isotope signature of the gas sources are in some cases not distinct enough, not temporally constant or are subject to changes on their way from source to examination area.